A behavior genetics research design is used to examine early precursors of adjustment and maladaptive behavior patterns in monozygotic and dizygotic twins. The twins are studied from late infancy through middle childhood in a collaborative study based in Colorado. The purpose is to assess genetic change and continuity in temperament, emotional organization, and cognition. Children are observed in structured and naturalistic situations in the home and laboratory. The specific goals of the NIMH component of the study are (1) to describe early development of prosocial and anti-social orientations, (2) to identify affective components that underlie these behaviors, and (3) to estimate the extent to which these early patterns of social-emotional development are heritable. Both mz and dz twins show highly synchronized, coordinated patterns of social interaction with their twin partners. They are equally prosocial and empathic to individuals outside the dyad, but patterns of generalization from the dyad to others differ for mz and dz twins. Empathy and guilt increase in frequency during the second year of life, while showing a decrease in genetic influence during this time period. In contrast, fear and anger are present earlier and show stronger evidence for heritability. Gender differences are present, particularly for empathy which is more apparent in girls.